Does the fuel line on the top of the can go back to the top of the tank? Does it flow fuel unrestricted? There should be no restriction in this return line.
Do you have a fuel filter? It should be on the inlet side of the fuel pump.
There is an air screw for idle mixture, right hand side of carb near the air horn, should be turned out about 1 turn. The more you turn it out, the leaner the idle mixture.
Get your kart to idle, adjust the slide screw, that’s the screw on the other side of the carb with a lock nut and a thumb screw head, back it out just to the point when the engine stalls. Now to restart, just a little throttle needs to applied.
Like William said, the vent hole in the cap is too small, mount a 1/4 inch barbed fitting and run a 1/4 inch ID hose into an overflow bottle above the fuel tank.
My jetting with a K&N style filter is 37.5 pilot, 195 main, my tracks are about 1,000 above sea level.
Hope this helps,
#55 Honda CR80
November 15-19, 2017
Las Vegas, Nevada
SKUSA SuperNationals 21
2018 Events TBA